Image sensor for filtering and coring pixel data and related methods

ABSTRACT

A method is for filtering data output from an array of pixels in an image sensor. The method may include filtering noise from variation in pixel response across the array. An electronic device may have a device for filtering data output from an array of pixels in an image sensor. The device may include a noise filter for removing variation in pixel response across the array.

FIELD OF THE INVENTION

The present invention relates to a digital filter, and in particular, toa digital filter that operates on data collected from a pixel array of asolid state image sensor.

BACKGROUND OF THE INVENTION

There is a continued desire for size reduction of solid-state cameramodules, notably in the consumer mobile market. However, space savinginnovations can sometimes have unwanted side effects. One of these sideeffects is a variation in pixel response across the array, which canarise from the design of the optical components or packaging, or fromthe design of the actual electronic sensor itself.

One aspect of electronic sensor design that may cause a variation inpixel response is the sharing of amplifying readout circuitry amongstgroups of pixels. Typically, readout circuitry may be shared between tworows of pixels. Because the angle of incidence of light impinging on thearray varies across the array, the readout circuitry provides a varying“shadowing” effect for different rows.

It is to be understood that the shadowing effect may not correspond to avisible shadow, as the dimensions are too small for this to be the case.The shadowing effect is instead to be thought of more generally as anyeffect that causes a decrease in the amount of light collected by thepixel under ideal operating conditions. For example, the shadowingeffect may be any means that blocks or impedes photons in their path tothe pixel, thus reducing the photon count over at least a portion of thepixel, or any means causing stray light gathering, that is, whereinlight that should be collected by one pixel is instead collected byanother.

This is illustrated in FIG. 1, which shows an array of pixels 10,arranged in rows R₁ to R_(n) and columns C₁ to C_(m). Successive rowscan be labeled as having an “odd” or an “even” parity, in accordancewith the parity of the row numbering. Readout circuitry 12 is provided,and as can be seen from the figure is shared between one even and oneodd row. The shadowing effect is illustrated by the regions 14-22, whichrepresent an area of each pixel 10 that is relatively poorly exposedwith respect to the remainder of the pixel. A lens (not shown) forprojecting an image onto the array is cantered at the center point ofthe array. It can be seen that on an upper portion 24 of the array, theodd rows R1 and R3 are compromised with a shadow effect.

The magnitude of the effect may be greatest at the uppermost edge of thearray, where the angle of incident light is the greatest with respect tothe normal to the plane of the pixel array. This is represented by therelatively larger area of the region 14 as compared with the region 16.At the center rows R5, R6, the readout circuitry 12 may have anegligible shadow effect 18. On a lower portion 26 of the array, theeven rows R8 and R10 are compromised with a shadow effect. Again, themagnitude of the effect is greatest at the uppermost edge of the array,where the angle of incident light is the greatest. This is representedby the relatively larger area of the region 22 as compared with theregion 20. Because the shadowing effect changes from odd rows to evenrows, it can be thought of as a signal with opposite phases on oppositesides of the pixel array.

Another aspect of electronic sensor design that may cause a variation inpixel response is the crosstalk between components that are introducedby a color filter array used in a color-sensitive image sensor. Toproduce a color sensitive image sensor, the pixel array is overlaid witha color filter array (CFA), so that each pixel is overlaid with amaterial with specific transmission characteristics so that the pixelcan be thought of as being responsive to a particular color. Usually onecolor is overlaid per pixel. As a short-hand notation, a “red” pixel isreferred to as a pixel that is sensitive to incident light having afrequency in the red part of the visible spectrum of light, with similarnotations used for other colors.

The specific frequency responses may vary according to the type ofmaterial used, among other factors. There are various well knownpatterns to use when depositing the color filter material. FIG. 2illustrates the common Bayer pattern, which comprises red, green, andblue sensitive pixels, labeled R, G, B respectively. Odd rows (R1, R3etc.) comprise alternate G and R pixels, and so can also be termed as“red” rows, while even rows (R2, R4 etc.) comprise alternate B and Gpixels, and so can be termed as “blue” rows.

A source of error in color sensitive image sensors is in the cross-talkof color components, that is, sometimes a photon that passes through aportion of the CFA sensitive to one color actually impacts on thephotocollection part of a pixel that is associated with another color.The photoelectric charge generated by the errant photon thereforecontributes towards the wrong color channel, providing a source ofnoise. A uniform crosstalk component may be present, arising from theactual construction of the pixel array.

However, because the CFA has a finite thickness and is overlaid on thepixel array, there may also be a component of crosstalk noise thatvaries according to the angle of incident light, providing a shadowingeffect in a similar fashion to that described above. The shadowingeffect is illustrated by the regions 30-48 in FIG. 2. Again, it can beseen that the magnitude of the shadowing effect increases towards theedges of the pixel array, and has a phase change associated withopposite sides of the array. Also, it can be seen that in respect to thered (odd) rows, the shadowing effect comprises the interference ofphotons that penetrate the blue (even) rows, while for the blue (even)rows, the shadowing effect comprises the interference of photons thatpenetrate the red (odd) rows.

It may be appreciated that most color-sensitive image sensors may alsoshare readout circuitry as shown in FIG. 1, and so the overall shadowingeffect for a given image sensor can arise as a combination of the sharedreadout circuitry and the crosstalk of color components, and it may beappreciated that there are other aspects of electronic sensor designthat contribute to this shadowing effect. The shadowing effect may alsovary according to different scene detail and scene luminance to beimaged by the pixel array.

It is also to be appreciated that only the row-to-row variation in pixelresponse has been discussed and illustrated with respect to FIGS. 1 and2. The pixel response may also vary in a similar fashion acrosssuccessive columns of the pixel array. This variation is not illustratedin FIGS. 1 and 2 for the convenience of illustration. The shadowingeffect when discussed in general is taken to include both row-to-row andcolumn-to-column variation.

The shadowing effect can be further exacerbated by the design of theoptical components or packaging. As camera modules become smaller, the“z-height” (the optical distance between lens and sensor surfaces) canbe reduced. However, this widens the angle of incident light incident onthe pixel array, and gives a greater variation of angle across thearray. This itself may cause a variation in pixel response, as thephotoelectric conversion of incident photons can be compromised if thephotons are incident at angles that cause the generated electrons to notbe collected by the charge collection wells. The increase in variationof angle can also serve to exacerbate the problems of variation of pixelresponse caused by the design of the actual electronic sensor itself, asdescribed above.

In digital sampling theory, the nyquist frequency is the maximumfrequency at which data can be reproduced. It corresponds to half thesampling rate of a digital system. For an image sensor, the nyquistfrequency represents the maximum achievable spatial distance of an imagethat can be reproduced, and is equal to one half the reciprocal of thecenter-to-center pixel spacing. Because it varies per pixel, row, orcolumn, the abovementioned shadowing effect therefore represents avariation of the pixel response at the nyquist frequency, which we nowterm for convenience as Nyquist Frequency Variation (NFV) of pixelresponse.

Currently, there may not be an image sensor that can cancel out NFV, asits effects have not previously been seen as significant when comparedwith other noise sources. However, with improvements in digitalfiltering, the effects of NFV are becoming more apparent to Applicant.The effect of NFV may in fact compromise the performance of key signalprocessing steps in the image reconstruction chain. One such step isnoise reduction.

Traditionally, innate noise sources in image sensors are wideband—eitherGaussian or impulsive—and techniques have been developed over the yearsto combat these unwelcome but well-understood defects of image data.However, NFV is narrowband, and its presence serves to alter thestatistics of innate noise sources, leading to structured noiseartifacts, which are much harder to separate from actual image detailand cancel. In color cameras, a further step (known as “demosaic”)interpolates missing data in the sensor signal to produce afully-sampled 3-channel color signal. Here NFV appears as a visible“linen” texture in areas of natural low-texture (e.g. blue sky), andbreaks up edge detail in the scene representation. Accordingly, theremay be a need for image sensors to cancel NFV.

SUMMARY OF THE INVENTION

According to a first aspect, there is provided a method of filteringdata output from an array of pixels in an image sensor. The methodcomprises the step of filtering noise caused by variations in pixelresponse across the array or across a portion of the array. The step offiltering noise caused by variations in pixel response comprisesobtaining a central pixel value and a plurality of neighboring pixelvalues, combining the neighboring pixel values to obtain an estimatepixel value co-sited with the central pixel value, comparing the centralpixel value with the estimate pixel value to obtain a difference signal,and coring the difference signal.

The step of coring the difference signal may be performed according to acoring level, which defines an amount by which difference values aredriven towards and not beyond zero. The coring level may be adjustable.The step of coring may further comprise the step of maintaining arunning estimate of NFV magnitude and adjusting the coring level inresponse to different NFV magnitudes. The coring level may be modulatedby the ratio of the estimated pixel value to the maximum signalmeasurable by a digital processing system that performs the filtering,to produce a weighted coring level.

In some embodiments, the step of combining the neighboring pixel valuesmay comprise calculating their average value. The step of combining theneighboring pixel values may comprise the steps of: group-summing aplurality of groups of pixel values, wherein different groups of pixelvalues are taken from pixels aligned in different directions, andcomparing the sums of two or more groups to determine at least onedirectional stripe value representing an image striped in a determineddirection.

The step of comparing the sums of the first and second groups of pixelvalues may comprise subtracting the sum of the group of pixel valuestaken from pixels aligned in the determined direction from the sum of asecond group of pixel values taken from pixels aligned in one or morechosen comparison directions. The comparison direction may be chosen tobe one direction normal to the determined direction.

An angle signal to be output may be determined based on choosing themaximum value from:

(1) one or more directional stripe values;

(2) one or more predetermined stripe threshold values corresponding to adifference in signal level above which two signals are considered asbelonging to different stripes, and

(3) a normalized stripe value based on comparison of the central pixelvalue with the sum of the neighboring pixels.

If the output image angle is a directional stripe value, the pixelvalues in the corresponding direction only may be combined in thecalculation of the estimate. Otherwise all the neighboring pixels may becombined in the calculation of the estimate. The different groups ofpixel values may comprise those taken from pixels aligned in opposingdiagonals. The opposing diagonals may be orthogonal and centered on andincluding the central pixel.

The different groups of pixel values may comprise those taken frompixels aligned in horizontal or vertical directions. The coring may beselectively applied based on a roughness measure, defined as thedifference between the maximum and minimum pixel values of theneighboring pixels. The coring may be applied if the roughness is withina set threshold.

The neighboring pixel values may be neighbors in a given color plane.The step of removing variations in pixel response may comprise applyinga notch filter. The notch filter may act to filter out signals at theimage sensor's nyquist frequency. The step of removing variations inpixel response may comprise applying an adaptive filter.

According to a second aspect, there is provided an apparatus forfiltering data output from an array of pixels in an image sensor,comprising means for filtering noise or a noise filter caused byvariations in pixel response across the array or across a portion of thearray. The apparatus may comprise read means or a readout circuit forobtaining a central pixel value and a plurality of neighboring pixelvalues, calculation means or a calculator for combining the neighboringpixel values to obtain an estimate pixel value co-sited with the centralpixel value, and a coring module comprising means for comparing or afirst comparator for comparing the central pixel value with the estimatepixel value to obtain a difference signal, and coring means or a coringprocessor for coring the difference signal.

The coring means may be adjustable to operate according to a coringlevel, which defines an amount by which difference values are driventowards and not beyond zero. The apparatus may further comprisemonitoring means or a monitor for maintaining a running estimate of NFVmagnitude and adjusting the coring level in response to different NFVmagnitudes.

The coring level can be modulated by the ratio of the estimated pixelvalue to the maximum signal measurable by a digital processing systemthat comprises the filtering apparatus to produce a weighted coringlevel. The calculation means may comprise an average calculator or meansfor calculating the average value of the neighboring pixel values.

The calculation means may comprise summation means or a summer forgroup-summing a plurality of groups of pixel values, wherein differentgroups of pixel values are taken from pixels aligned in differentdirections, and a second comparator or comparison means for comparingthe sums of two or more groups to determine at least one directionalstripe value representing an image striped in a determined direction.

The comparison means may comprise subtraction means or a subtractor forsubtracting the sum of the group of pixel values taken from pixelsaligned in the determined direction from the sum of a second group ofpixel values taken from pixels aligned in one or more chosen comparisondirections. The comparison direction may be chosen to be one directionnormal to the determined direction. The apparatus may further compriseangle determination means or an angle determiner comprising rank sortmeans or a rank sorter adapted to sort at least:

(1) one or more directional stripe values;

(2) a predetermined stripe threshold value corresponding to a differencein signal level above which two signals are considered as belonging todifferent stripes; and

(3) a normalized stripe value based on comparison of the central pixelvalue with the sum of the neighboring pixels.

The apparatus may further comprise selection means or a selector forselecting the maximum value and outputting that maximum value as thedetermined angle value. The apparatus may comprise decision means or alogic device for selecting a method of calculating the estimate pixelvalue, wherein, if the output image angle is a directional stripe value,the pixel values in the corresponding direction may be only combined inthe calculation of the estimate; otherwise all the neighboring pixelsmay be combined in the calculation of the estimate.

The different groups of pixel values may comprise those taken frompixels aligned in opposing diagonals. The opposing diagonals may beorthogonal and centered on and including the central pixel. Thedifferent groups of pixel values may comprise those taken from pixelsaligned in horizontal or vertical directions.

In some embodiments, the apparatus may further comprise signal roughnessmeasurement means or a roughness measurer arranged to determine thedifference between the maximum and minimum pixel values of theneighboring pixels, and switch means or a switch for selectivelyignoring the output of the coring means based on the measured roughness.

The neighboring pixel values may be neighbors in a given color plane.The means or a noise filter for filtering noise caused by variations inpixel response may comprise a notch filter. The notch filter can act tofilter out signals at the image sensor's nyquist frequency. The meansfor filtering noise caused by variations in pixel response may comprisean adaptive filter.

According to further aspects, an image sensor comprises the apparatus ofthe second aspect, and a mobile communications device, optical pointingdevice, or web cam comprise the image sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may now be described, by way of example only, withreference to the accompanying drawings, in which:

FIG. 1 illustrates an image sensor pixel array wherein readout circuitryis shared between rows, according to the prior art;

FIG. 2 illustrates the Bayer pattern color filter array, according tothe prior art;

FIG. 3 illustrates the operation of a coring filter according to thepresent invention;

FIG. 4 schematically illustrates a digital filter according to a firstembodiment of the present invention;

FIG. 5 schematically illustrates a digital filter according to a secondembodiment of the present invention; and

FIG. 6 illustrates the process of an aspect of a filtering stepaccording to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As mentioned above, noise caused by variation of pixel response acrossthe array of pixels may be a drawback of known image sensors. Becausethe NFV is narrowband, a first possible approach would be to deploy afixed notch-filter at the frequency of variation of response, which inmost cases may be the nyquist frequency. It can also be set at a chosenother frequency for special case image sensors.

While this approach may be the preferred option for someimplementations, there may be some problems associated with it. In theworld of video and stills imaging, an effective notch-filter isexpensive, and loss of high-frequency data (blurring) is the inevitableresult of a practical filter design. For color image processing, ifnotch-filtering is performed before the demosaic step, the Nyquistregion is relatively smaller and blurring is intensified. If performedafter demosaic, any prior noise reduction is compromised as describedearlier.

A further possible option that may mitigate the blurring effect is theuse of an adaptive filter. Again, this approach may be preferred forsome applications but in general presents some difficulties: it isdifficult to separate noise and data in regions of high luminanceactivity. It is difficult to develop a model on which adaptation canoccur, other than in regions of low textural activity where thenotch-filter would bring good results anyway. Thus, the NFV tends tosurvive around object edges in the scene, and cancellation is onlypartially effective.

A further weakness of the adaptive filtering approach may be that thenotch-filter may be minimum-phase, to allow fade-in and fade-out withoutphase-shifting. A minimum-phase notch-filter at Nyquist is difficult toachieve (the fixed-response filter does not suffer from thisrestriction, as a phase-shift by a fixed distance is perfectlyacceptable over an entire image).

A more advanced option that avoids these problems is the use of coring.Coring is a technique, which in itself is well known. It is applied tosignal differences to drive differences by a certain amount towards, butnot beyond, zero. The operation of a coring function is shown in FIG. 3,which shows how an input value plotted on the x-axis is changed to anoutput value, plotted on the y-axis. The line 50 represents an unchangedsignal, where the output is equal to the input, while the line 52illustrates the operation of a coring function. In the illustratedexample, input signals with a value above 1 are driven by a value of 1towards zero. The values between −1 and 1 are removed. The amount bywhich values are driven towards zero is herein termed the “coringlevel”. It is also sometimes known as the coring depth.

Coring does not involve neighboring pixel data, although neighboringdata may have contributed to the formation of either or both of the twosignals whose difference is subject to coring. Thus coring may be onlyapplicable to minimum phase (co-sited) signal pairs. The function ofcoring is to drive differences by a certain amount towards, but notbeyond, zero.

A common example of coring can be found in the peaking, or sharpening,of images. This signal processing step may be usually performed late inthe image reconstruction chain giving a bandpass or highpass boost toimage data, compensating for upstream lowpass mechanisms. As residualnoise is also boosted by this process, coring can act as an effectivelate-stage noise reduction process, while still passing the largersignal swings which serve to sharpen image detail. Level settings may besubtle if visible image degradation is to be avoided, thus coring is notusually deployed in normal (low-noise) operation.

Cored peaking is only employed part of the time during operation ofexisting image sensors, and under normal operating conditions is onlyemployed with subtle coring levels. Furthermore, peaking comes afternoise-reduction and demosaic in the reconstruction chain, by which timethe damage is done.

The use of peaked coring for NFV suppression has not previously beenconsidered because the actual problem of how to remove noise caused byvariation of pixel response across the array of pixels has notpreviously been considered. Furthermore, the coring levels for NFVsuppression are much higher than that of cored peaking and may always bedeployed. So the use of cored peaking for NFV suppression would be seenas too little, too late.

However, Applicant has realized that a coring method can in fact beapplied to the NFV problem. This may hereafter be referred to as“nyquist coring”. In this case, the model of NFV is a gain error, andthe coring level is made level-dependent, i.e. it is expressed as apercentage of the signal value. We may refer to this percentage value asthe “adaptive coring level”. It can be varied within an image with noloss of generality. The adaptive coring level can further be modulatedby the co-sited signal estimate to produce a “weighted coring level”used in the coring process.

This coring process may now be discussed with reference to FIGS. 4 and5. As a first step, data is co-sited, so that the minimum phase signalpairs are generated. The example shown in FIG. 4 shows an application toneighboring green pixels in a Bayer pattern. The central pixel 54 hasfour neighbors (ring pixels), which for convenience are labeled asnorth-west, north-east, south-west and south-east neighbors 56, 58, 60,62, respectively. Note that if the center pixel is sited on an even row,the neighbors are sited on odd row and vice-versa, the same applies tocolumns. The values of the ring pixels are output to a cositing filter64, which operates on them to output an estimate of the central pixelvalue in the absence of NFV.

The estimate and the actual value from the central pixel site are inputto a nyquist coring module 66. The difference between this estimate andthe actual value at the site then represents the NFV, and is cored-outif below the set coring level.

The scope of the invention is not limited to any particular cositingfilter design. One possible option is to simply calculate the average ofthe neighbors. However, an improved version can make use of an adaptivegradient analysis, which is illustrated in FIG. 5. An example gradientanalysis means or a gradient analyzer 68 is shown in FIG. 5. The sum ofeach diagonal and the sum of all the ring pixels are calculated. Anorthogonal stripe value is calculated, which is defined as the absolutevalue of {(4*central pixel value)−(the ring sum)}. This value may besmall if the central pixel value 54 is close to the average of itsneighboring pixels 56-62. The orthogonal stripe value is orthogonal andnormalized.

A “stripe” in the north east direction is then calculated, defined asthe absolute value of {(2*central pixel value)+(sum of north east pixel58 and south west pixel 60)−(sum of north west pixel 56 and south eastpixel 60)} and a “stripe” in the south east direction is calculated,defined as the absolute value of {(2*central pixel value)+(sum of northwest pixel 56 and south east pixel 60)−(sum of north east pixel 58 andsouth west pixel 60)}.

It can be seen that if the sum of the north-east and south-west pixels58, 60 is greater than the sum of the north-west and south-east pixels56,62, then the value of the north-east stripe may be greater than thevalue of the south-east stripe. Alternatively, if the sum of thenorth-east and south-west pixels 58,60 is less than the sum of thenorth-west and south-east pixels 56, 62, then the value of thenorth-east stripe may be less than the value of the south-east stripe.

Furthermore, a null stripe value is also defined based on apredetermined stripe threshold. This determines the sensitivity of thegradient analysis by setting an arbitrary level for the stripedetectors, above which it is considered a stripe to be detected. Theorthogonal stripe, north-east stripe, south-east stripe and the stripethreshold are then sorted by magnitude. If the maximum value is thenorth-east stripe, the gradient analysis means 68 outputs a “NE” anglesignal, while if the maximum value is the south-east stripe, thegradient analysis means 68 outputs a “SE” angle signal. Otherwise, theangle signal is output as a null value, indicating that no discernablegradient is present.

The output angle signal is then input to an adaptive cositing filtermeans or a cositing filter (70 in FIG. 5). If an “NE” angle is input,the adaptive cositing filter 70 uses the average of the north east andsouth west pixels 58,60 as the estimate to be output, while if a “SE”angle is input, the adaptive cositing filter 70 uses the average of thenorth-west and south-east pixels 56, 62 as the estimate to be output.However, if the angle is a null value, the estimate is calculated as theaverage value of the ring pixels 56-62.

It is to be appreciated that the above is only an example, and that thegradient analysis is not limited to comparison of only two diagonals.The stripe value in any determined direction can be found by comparisonwith any other direction, termed as a comparison direction. Thecomparison direction may usually be normal to the determined direction,but it does not need to be so. Also, as well as diagonal stripes thatinclude the central pixel value, the stripes could be off-diagonalangles that include the central pixel value, other directions that donot include the central pixel value, and horizontal and/or verticaldirections.

Next, the scale of the problem (how great is the maximum expected NFV ata given point) may be estimated throughout the image. As stated earlier,this is a complicated function of spatial index and of scene data. In asimple configuration a coring level 72, representing the envisagedworst-case NFV, can be set externally. However, this may inevitablyresult in over-attenuation of scene detail at the Nyquist frequency.Another approach is to maintain an internal estimate of NFV whichmaintains a running estimate of NFV magnitude and adjusts the coringlevel in response to different NFV conditions, as might be encounteredin different regions of an image as described earlier. This allows anacceptable margin to be maintained between transmission of NFV andattenuation of scene detail.

An alternative and possibly simpler means to the same end is to use a“roughness” measure, derived from neighboring data (e.g. the differencebetween the brightest and darkest pixel) to inhibit coring inhighly-textured regions. Here NFV is less noticeable, for example, “outin the open” (blue sky), where suppression is mandatory. A roughnessmeasurement means or a roughness measurer 74 is illustrated in FIG. 5.It has the ring pixel values 56-62 and an adjustable ceiling level 76 asits inputs, in order to give the above-mentioned “roughness” output. Theroughness measure is defined as the difference between the maximum andminimum pixel values, which can for example be determined after thevalues are sorted. If the roughness is found to be greater than theceiling value, the ceiling value is output. Otherwise, the calculatedroughness value is output.

The nyquist coring is carried out by coring means 66, on the basis ofthe cosited pixel data compared with the central pixel value 54. Asmentioned above, the coring level 72 can further be modulated by theco-sited signal estimate to produce a “weighted coring level” used inthe coring process.

An example coring process is illustrated in FIG. 6. A weighted coringlevel can be defined as the {(estimate obtained from the cosited data)divided by (the maximum signal for a given system reapproach)* coringlevel}. A “difference” is then defined as the (estimate)−(the centralpixel value). The cored difference is then set to either the“difference” or the weighted coring level, in accordance with the flowchart illustrated in FIG. 6. The nyquist cored value for the centralpixel is then set at the center pixel value + the cored difference.Finally, a soft switch means or a soft switch 78 (FIG. 5) operates tooutput a weighted sum of the original central pixel value and thenyquist cored central pixel value, the relative weights based on inputsfrom the roughness measurement 74, gradient analysis 68, central pixeland the nyquist cored central pixel output.

The filter of this invention can be implemented in an image sensor andin any device incorporating the image sensor, for example, a digitalstill image camera, mobile communications device, optical pointingdevice such as an optical mouse, or a webcam, among others. Variousimprovements and modifications may be made to the above withoutdeparting from the scope of the invention. For example, the various“means” for performing the functions of the invention may typicallycomprise electrical circuits that are either programmed or programmableto implement digital filtering and signal processing algorithms. Anycombination of hardware, firmware or software may be chosen asconvenient. However, it may also be appreciated that the physicalmechanisms may comprise other types of circuit or other types oftransmission, in well known manners.

1. A method of filtering data output from an array of pixels in an imagesensor comprising: filtering noise from a variation in pixel responseacross the array of pixels in the image sensor by at least performingobtaining a central pixel value and a plurality of neighboring pixelvalues, combining the plurality of neighboring pixel values to obtain anestimated pixel value co-sited with the central pixel value, comparingthe central pixel value with the estimated pixel value to provide adifference signal, and coring the difference signal using processor. 2.The method of claim 1 wherein the variation comprises Nyquist FrequencyVariation (NFV).
 3. The method of claim 1 wherein coring the differencesignal is performed according to a coring level, the coring leveldefining an amount by which difference values are driven towards and notbeyond zero.
 4. The method of claim 3 wherein the coring level isadjustable.
 5. The method of claim 4 wherein coring the differencesignal comprises maintaining a running estimate of NFV magnitude andadjusting the coring level in response to different NFV magnitudes. 6.The method of claim 3 wherein the coring level is modulated by a ratioof the estimated pixel value to a maximum signal measurable by a digitalprocessing system that performs the filtering to produce a weightedcoring level.
 7. The method of claim 1 wherein combining the pluralityof neighboring pixel values comprises calculating an average value ofthe plurality of neighboring values.
 8. The method of claim 1 whereincombining the plurality of neighboring pixel values comprises: summing aplurality of groups of pixel values to produce at least a first andsecond summed group of pixel values, each summed group of pixel valuescomprising pixels aligned in different directions; and comparing atleast the first and the second summed group of pixel values to determineat least one directional stripe value representing an image striped in adetermined direction.
 9. The method of claim 8 wherein comparing atleast the first and the second summed group of pixel values comprisessubtracting the first summed group of pixel values comprising pixelsaligned in the determined direction from the second summed group ofpixel values taken from pixels aligned in at least one comparisondirections.
 10. The method of claim 9 wherein the comparison directioncomprises a direction normal to the determined direction.
 11. The methodof claim 9 wherein an angle signal to be output is determined based uponchoosing a maximum value from at least: at least one directional stripevalues; at least one predetermined stripe threshold value correspondingto a difference in signal level above which two signals are consideredas belonging to different stripes; and a normalized stripe value basedon comparison of the central pixel value with a sum of the neighboringpixels.
 12. The method of claim 11 wherein if an output image angle is adirectional stripe value, the pixel values in the correspondingdirection are combined in the calculation of the estimated pixel value;and wherein if an output image angle is not the directional stripevalue, all the neighboring pixels are combined in a calculation of theestimated pixel value.
 13. The method of claim 8 wherein each summedgroup of pixel values comprises pixels aligned in opposing diagonals.14. The method of claim 13 wherein the opposing diagonals are orthogonaland centered on and including the central pixel.
 15. The method of claim8 wherein each summed group of pixel values comprises pixels aligned inhorizontal or vertical directions.
 16. The method of claim 1 whereincoring the difference signal comprises selectively coring based upon aroughness measure, being a difference between maximum and minimum pixelvalues of the neighboring pixels.
 17. The method of claim 16 whereincoring the difference signal comprises coring the difference signal whenthe roughness measure is within a set threshold.
 18. The method of claim1 wherein the neighboring pixel values comprise neighboring pixel valuesin a given color plane.
 19. The method of claim 1 wherein filteringnoise from variations in pixel response comprises applying a notchfilter.
 20. The method of claim 19 wherein the notch filter acts tofilter out signals at a Nyquist frequency of the image sensor.
 21. Themethod of claim 1 wherein filtering noise from variations in pixelresponse comprises applying an adaptive filter.
 22. An electronic devicehaving a device for filtering data output from an array of pixels in animage sensor, the device comprising: a noise filter configured to removevariation in pixel response across the array; a readout circuitconfigured to obtain a central pixel value and a plurality ofneighboring pixel values; a calculator configured to combine theplurality of neighboring pixel values for obtaining an estimated pixelvalue co-sited with the central pixel value; and a coring modulecomprising a first comparator configured to compare the central pixelvalue with the estimated pixel value for producing a difference signal,and a coring processor configured to core the difference signal.
 23. Theelectronic device of claim 22 wherein the variation comprises NyquistFrequency Variation (NFV).
 24. The electronic device of claim 22 whereinsaid coring processor is configured to be adjustable to operateaccording to a coring level, defining an amount by which differencevalues are driven towards and not beyond zero.
 25. The electronic deviceof claim 24 further comprising a monitor configured to maintain arunning estimate of NFV magnitude and adjusting the coring level inresponse to different NFV magnitudes.
 26. The electronic device of claim25 wherein the coring level can be modulated by the ratio of theestimated pixel value to a maximum signal measurable by a digitalprocessing system that comprises the filtering electronic device toproduce a weighted coring level.
 27. The electronic device of claim 22wherein said calculator comprises an average calculator configured toprovide an average value of the plurality of neighboring pixel values.28. The electronic device of claim 22 wherein said calculator comprises:a summer configured to sum a plurality of groups of pixel values toproduce at least a first and second summed group of pixel values, eachsummed group of pixel values comprising pixels aligned in differentdirections; and a second comparator configured to compare at least thefirst and the second summed group of pixel values to determine at leastone directional stripe value representing an image striped in adetermined direction.
 29. The electronic device of claim 28 wherein saidsecond comparator comprises a subtractor configured to subtract thefirst summed group of pixel values comprising pixels aligned in thedetermined direction from the second summed group of pixel values takenfrom pixels aligned in at least one comparison directions.
 30. Theelectronic device of claim 29 wherein the at least one comparisondirection comprises a direction normal to the determined direction. 31.The electronic device of claim 30 further comprising: an angledeterminer having a rank sorter configured to sort at least the at leastone directional stripe values; at least one predetermined stripethreshold value corresponding to a difference in signal level abovewhich two signals are considered as belonging to different stripes; anda normalized stripe value based on comparison of the central pixel valuewith a sum of the neighboring pixels; and a selector configured toselect a maximum value and outputting the maximum value as a determinedangle value.
 32. The electronic device of claim 31 further comprising alogic device configured to select a method of calculating the estimatedpixel value; and wherein if an output image angle is a directionalstripe value, the pixel values in the corresponding direction only arecombined in the calculation of the estimated pixel value; and wherein ifthe output image angle is not the directional stripe value, all theneighboring pixels are combined in a calculation of the estimated pixelvalue.
 33. The electronic device of claim 28 wherein each summed groupof pixel values comprises pixels aligned in opposing diagonals.
 34. Theelectronic device of claim 33 wherein the opposing diagonals areorthogonal and centered on and including the central pixel.
 35. Theelectronic device of claim 28 wherein each summed group of pixel valuescomprises pixels aligned in horizontal or vertical directions.
 36. Theelectronic device of claim 22 further comprising: a signal roughnessmeasurer configured to determine a difference between maximum andminimum pixel values of the plurality of neighboring pixels; and aswitch configured to selectively ignore an output of said coringprocessor based on a measured roughness.
 37. The electronic device ofclaim 22 wherein the plurality of neighboring pixel values comprisesneighbors in a given color plane.
 38. The electronic device of claim 22wherein said noise filter comprises a notch filter.
 39. The electronicdevice of claim 38 wherein said notch filter is configured to filter outsignals at a Nyquist frequency of the image sensor.
 40. The electronicdevice of claim 22 wherein said noise filter comprises an adaptivefilter.
 41. The electronic device of claim 22 wherein the electronicdevice comprises at least one of a mobile communications device, anoptical pointing device, and a web cam.
 42. An image sensor comprising adevice for filtering data output from an array of pixels in the imagesensor, the device comprising: a noise filter for removing variations inpixel response across the array; a readout circuit for obtaining acentral pixel value and a plurality of neighboring pixel values; acalculator for combining the plurality of neighboring pixel values toobtain an estimated pixel value co-sited with the central pixel value;and a coring module comprising a first comparator for comparing thecentral pixel value with the estimated pixel value to produce adifference signal, and a coring processor for coring the differencesignal.
 43. The image sensor of claim 42 wherein said coring processoris adjustable to operate according to a coring level, defining an amountby which difference values are driven towards and not beyond zero. 44.The image sensor of claim 43 further comprising a monitor formaintaining a running estimate of Nyquist Frequency Variation (NFV)magnitude and adjusting the coring level in response to different NFVmagnitudes.
 45. The image sensor of claim 42 wherein said noise filtercomprises a notch filter.
 46. A mobile communications device having animage sensor and a device for filtering data output from an array ofpixels in the image sensor, the device comprising: a noise filter forremoving variations in pixel response across the array; a readoutcircuit for obtaining a central pixel value and a plurality ofneighboring pixel values; a calculator for combining the plurality ofneighboring pixel values to obtain an estimated pixel value co-sitedwith the central pixel value; and a coring module comprising a firstcomparator for comparing the central pixel value with the estimatedpixel value to produce a difference signal, and a coring processor forcoring the difference signal.
 47. The mobile communications device ofclaim 46 wherein said coring processor is adjustable to operateaccording to a coring level, defining an amount by which differencevalues are driven towards and not beyond zero.
 48. The mobilecommunications device of claim 47 further comprising a monitor formaintaining a running estimate of Nyquist Frequency Variation (NFV)magnitude and adjusting the coring level in response to different NFVmagnitudes.
 49. An optical pointing device having an image sensor and adevice for filtering data output from an array of pixels in the imagesensor, the device comprising: a noise filter for removing variations inpixel response across the array; a readout circuit for obtaining acentral pixel value and a plurality of neighboring pixel values; acalculator for combining the plurality of neighboring pixel values toobtain an estimated pixel value co-sited with the central pixel value;and a coring module comprising a first comparator for comparing thecentral pixel value with the estimated pixel value to produce adifference signal, and a coring processor for coring the differencesignal.
 50. The optical pointing device of claim 49 wherein said coringprocessor is adjustable to operate according to a coring level, definingan amount by which difference values are driven towards and not beyondzero.
 51. The optical pointing device of claim 50 further comprising amonitor for maintaining a running estimate of Nyquist FrequencyVariation (NFV) magnitude and adjusting the coring level in response todifferent NFV magnitudes.
 52. A web cam having an image sensor and adevice for filtering data output from an array of pixels in the imagesensor, the device comprising: a noise filter for removing variations inpixel response across the array; a readout circuit for obtaining acentral pixel value and a plurality of neighboring pixel values; acalculator for combining the plurality of neighboring pixel values toobtain an estimated pixel value co-sited with the central pixel value;and a coring module comprising a first comparator for comparing thecentral pixel value with the estimated pixel value to produce adifference signal, and a coring processor for coring the differencesignal.
 53. The web cam of claim 52 wherein said coring processor isadjustable to operate according to a coring level, defining an amount bywhich difference values are driven towards and not beyond zero.
 54. Theweb cam of claim 53 further comprising a monitor for maintaining arunning estimate of Nyquist Frequency Variation (NFV) magnitude andadjusting the coring level in response to different NFV magnitudes.